Paper and paperboard materials used in packaging applications require barrier properties to protect dry goods in the package interior from moisture in the external environment. Barrier coatings for paper substrates, which have been described in prior art, can impart water resistance, grease resistance, or chemical resistance. They can diminish water vapor transmission rates through paper. Barrier coatings can also prevent other undesired chemical migration through the substrate. These barrier coating functions are especially useful for many paper packaging applications for food and non-food packaging applications. Packaging may be required to protect dry goods and powdered materials from the ingress of moisture in humid conditions. Conversely, disposable paper cups are required to retain their liquid contents. In some cases, a barrier coating applied during the process of manufacturing the paper may be sufficient to meet these packaging requirements, but such coated materials also have limitations, which are widely understood. For instance, polyolefin based wax materials can provide effective barriers when coated or laminated on paper, but such wax coatings have low gloss, high coating weight, and impede the recyclability of the paper. The barrier treatments that are applied during paper manufacturing may also adversely affect subsequent processes, such as printing, stamping, or gluing paper packages.
Prior art also teaches examples of effective printable barrier coatings on paper that can be applied to selected areas of a package before or after printing inks onto the substrate by offset printing, flexographic printing, ink jet printing or other means. Such barrier coatings known in the art may be constructed as solvent-based coatings, water-based coatings, or 100% solid coatings. It is well known in the art that the incorporation of materials with a hydrophobic, cycloaliphatic backbone into coatings improves their intrinsic barrier properties, especially by lowering the permeability of water vapor (See for example J. M. Oliver, D. S. Babcock, “Influences on Barrier Performance of UV/EB Cured Polymers”, RadTech 2012 Technical Conference). Factors thought to contribute to the performance of cycloaliphatic materials in barrier materials are (a) the hydrophobicity of the cycloaliphatic backbone containing alicyclic hydrocarbon groups (reducing water vapor solubility), (b) the high crosslink density achieved in cured materials (reducing water vapor diffusion rate), and (c) rigid alicyclic backbone structures with low segment mobility (also reducing the permeant diffusion rate).
It is also well known that hydrophobic wax materials can be incorporated into barrier coatings to increase water resistance and to lower the permeability of water vapor. However, methods in prior art that teach this use of hydrophobic wax materials do not address the dull surface appearance and low gloss that accompanies their use when incorporated into coating compositions. The appearance and gloss of radiation curable coatings is related to the poor solubility and crystallinity of hydrophobic wax materials. Wax materials typically fail to form solutions or homogeneous dispersions in the coating matrix. Poorly compatible wax materials accumulate at the coating surface where they form crystals and amorphous aggregates that diminish the specular reflection contributing to gloss. Low refractive indices of hydrophobic wax materials also cause a diminishment of coating gloss.
Gloss is an important, highly desired property of radiation curable coatings for paper packaging applications. A clear coating with a high level of gloss makes underlying printed images appear more distinct and colorful. The achievement of high gloss is enabled by the high solid level in radiation curable coating, which may approach 100 wt. % of non-volatile solids. The high solid level contributes to the ability of radiation curable coatings to form a continuous coating with a smooth surface on top of the paper surface. It may be highly desirable for some applications related to paper packaging to design radiation curable coatings that function in a dual role as a both a barrier coating and a gloss coating. The incorporation of hydrophobic waxes to lower MVTR of the coating forces a trade-off between barrier performance and gloss.
The prior art discloses other barrier coatings that contain water or organic solvents. These other coating types require drying capability on the printing press to remove the water or solvents from the printed coating. The generation of volatile organic compounds from solvents in the printing process is also regulated and restricted by environmental standards in some places. The absence of water or volatile solids in the coating of the present invention is a technical advantage. Some of the of barrier materials containing water or organic solvents also are incapable of being applied to paper or paperboard in a conventional flexographic printing process because they may lack appropriate rheology or chemistry that would be compatible with the flexographic printing process. The coatings of the present invention can be applied selectively to areas of substrate that require it; whereas these other barrier materials must be applied uniformly all over the paper or paperboard. The printable barrier coatings of the present invention can be applied over printed inks or surface features that may be produced on the substrate surface during construction of a paper package. Printability is an important technical advantage.
The prior art also discloses hot-melt barrier and molten wax barriers, which need not contain any water or volatile solvents; however, these types of coatings are solid materials at room temperature. They must be heated above room temperature to melt them; thus, these materials are also not compatible with a conventional flexographic printing process. The coatings of the present invention remain liquid until after printing onto the paper or paper board in selected areas and curing by actinic radiation. Hot-melt and molten wax barriers on paper may also have other deficiencies, such as low gloss, dull hazy appearance, poor adhesion, and slipperiness.
The prior art also discloses laminated polyethylene film that can be used as a barrier layer for paper packaging materials; however, the lamination process applies the barrier film uniformly all over paper or paperboard surfaces. The laminated barrier film cannot be applied selectively to the substrate in a flexographic printing process or by any other envisioned print process.
Some barrier coatings disclosed in prior art contain halogenated polymers or halogenated waxes. The halogenated coating materials have undesired environmental impact and often require special material disposal. The coating compositions of the present invention do not rely upon the use of any halogenated organic materials.
The present invention relates to a printable radiation curable high gloss barrier coating composition. The main technical advantage of this invention is discernible after the coating is printed onto the ink receptive surface of paper or paperboard and then exposed to actinic radiation. The substrate printed with this composition achieves a combination of properties, consisting of a low MVTR and high gloss, not achievable by other conventional radiation curable coatings. Prior art discloses other printable radiation curable barrier coating compositions, but none of these other compositions claim a combination of low MVTR and high gloss when printed on paper or paperboard.
The present invention describes printable barrier coating compositions for paper packaging that include the following components: one or more hydrophobic, cycloaliphatic radiation-curable monomers; one or more alcohol-functional waxes or sterols; and one or more additional hydrophobic waxes. After being printed and cured on the ink receptive surface of paper or paperboard, the printed article has a low MVTR, and the cured surface has improved and higher gloss due to the alcohol-functional material. Prior art discloses other printable barrier coating compositions that are radiation curable (e.g. WO 2012/061704, discloses a ream wrap material with a radiation curable barrier coating). Prior art also discloses compositions that contain hydrophobic, cycloaliphatic, radiation-curable monomers that are used in barrier materials (e.g. US 2007/0117917 and WO 2006/107802 describe barrier sealants for electronic materials); therefore the use of hydrophobic, cycloaliphatic monomers in barrier materials is generally known in the art. The use of hydrophobic wax materials in barrier coatings is also generally known (e.g. U.S. Pat. No. 7,915,183, describing a paraffin wax coating on paper). The unexpected discovery of this invention is that an alcohol-functional wax or sterol combined with the cycloaliphatic monomers and an additional hydrophobic wax material, when cured on paper, produces an excellent barrier coating with improved, higher gloss. High gloss is a desired property with added benefit for the paper packaging application, and the incorporation of hydrophobic wax material typically diminishes gloss. This invention discloses the use of an alcohol-functional wax or sterol to increase the gloss of the barrier coating.